Among the various video display systems available in the art, an optical projection system is known to be capable of providing high quality displays in a large scale. In such an optical projection system, light from a lamp is uniformly illuminated onto an array of, e.g., M.times.N, actuated mirrors, wherein each of the mirrors is coupled with each of the actuators. The actuators may be made of an electrodisplacive material such as a piezoelectric or an electrostrictive material which deforms in response to an electric field applied thereto.
The reflected light beam from each of the mirrors is incident upon an aperture of, e.g., an optical baffle. By applying an electric signal to each of the actuators, the relative position of each of the mirrors to the incident light beam is altered, thereby causing a deviation in the optical path of the reflected beam from each of the mirrors. As the optical path of each of the reflected beams is varied, the amount of light reflected from each of the mirrors which passes through the aperture is changed, thereby modulating the intensity of the beam. The modulated beams through the aperture are transmitted onto a projection screen via an appropriate optical device such as a projection lens, to thereby display an image thereon.
In FIGS. 1A to 1I, there are illustrated manufacturing steps involved in manufacturing an array 200 of M.times.N thin film actuated mirrors 201, wherein M and N are integers, disclosed in a copending commonly owned application, U.S. Ser. No. 08/704,340, entitled "THIN FILM ACTUATED MIRROR ARRAY FOR USE IN AN OPTICAL PROJECTION SYSTEM".
The process for the manufacture of the array 200 begins with the preparation of an active matrix 110 including a substrate 112 with an array of M.times.N switching devices, e.g., metal-oxide-semiconductor (MOS) transistors 115 and a field oxide layer 116 formed on top thereof. Each of the MOS transistors 115 has a source/drain region 117, a gate oxide layer 118 and a gate electrode 119.
In a subsequent step, there is deposited a first passivation layer 120, made of, e.g., PSG or silicon nitride, and having a thickness of 0.1-2 .mu.m, on top of the active matrix 110 by using, e.g., a CVD or a spin coating method.
Thereafter, an etchant stopping layer 130, made of a nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the first passivation layer 120 by using, e.g., a sputtering or a CVD method, as shown in FIG. 1A.
Then, a thin film sacrificial layer 140 is formed on top of the etchant stopping layer 130. The thin film sacrificial layer 140 is formed by using a sputtering or an evaporation method if the thin film sacrificial layer 140 is made of a metal, a CVD or a spin coating method if the thin film sacrificial layer 140 is made of a PSG, or a CVD method if the thin film sacrificial layer 140 is made of a poly-Si.
Subsequently, an array of M.times.N empty cavities (not shown) is created on the thin film sacrificial layer 140, in such a way that each of the empty cavities encompasses the source/drain region 117 in each of the MOS transistors 115, by using a dry or an wet etching method.
In a next step, an elastic layer 150, made of an insulating material, e.g., silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited on top of the thin film sacrificial layer 140 including the empty cavities by using a CVD method.
Thereafter, a second thin film layer 160, made of an electrically conducting material, e.g., Pt/Ta, and having a thickness of 0.1-2 .mu.m, is formed on top of the elastic layer 150 by using a sputtering or a vacuum evaporation method. The second thin film layer 160 is then iso-cut in a columnar direction by using an etching method, as shown in FIG. 1B.
Then, a thin film electrodisplacive layer (not shown), made of a piezoelectric material, e.g., PZT, or an electrostrictive material, e.g., PMN, and having a thickness of 0.1-2 .mu.m, is deposited on top of the second thin film layer 160 by using an evaporation, a Sol-Gel, a sputtering or a CVD method.
Next, the thin film electrodisplacive layer is patterned into an array of M.times.N thin film electrodisplacive members 175 by using a photolithography or a laser trimming method, as shown in FIG. 1C.
In a subsequent step, the second thin film layer 160 and the elastic layer 150 are, respectively, patterned into an array of M.times.N second thin film electrodes 165 and an array of M.times.N elastic members 155 by using an etching method, as shown in FIG. 1D.
In an ensuing step, portions of the etchant stopping layer 130 and the first passivation layer 120 formed on top of the source/drain region 117 in each of the MOS transistors 115, are selectively removed, while leaving intact portions 125 thereof surrounding the gate electrode 119 and the gate oxide layer 118 in each of the MOS transistors 115, by using an etching method, as shown in FIG. 1E.
Subsequently, an array of M.times.N first thin film electrodes 185 and an array of contact members 183 are formed by: first forming a layer (not shown), made of an electrically conducting material, completely covering the above structure, using a sputtering or a vacuum evaporation method; and then selectively removing the layer, using an etching method, as shown in FIG. 1F. Each of the first thin film electrodes 185 is located on top of the thin film electrodisplacive member 175. Each of the contact members 183 is positioned in such a way that it electrically connects the second thin film electrode 165 with the source/drain region 117 in each of the MOS transistors 115.
In a following step, a second passivation layer 187, made of, e.g., PSG or silicon nitride, and having a thickness of 0.1-2 .mu.m, is deposited by using, e.g., a CVD or a spin coating method, and then is patterned in such a way that it completely covers the contact members 183, by using an etching method, thereby forming an array 210 of M.times.N actuated mirror structures 211, as shown in FIG. 1G.
The preceeding step is then followed by completely covering each of the actuated mirror structures 211 with a first thin film protection layer (not shown).
The thin film sacrificial layer 140 is then removed by using an etching method. Thereafter, the first thin film protection layer is removed, thereby forming an array of M.times.N actuating structures 100, each of the actuating structures 100 having a proximal and a distal ends (not shown), as shown in FIG. 1H.
In a next step, the array of M.times.N actuating structures 100 is covered with a sacrificial material, including the spaces formed when the thin film sacrificial layer 140 was removed, in such a way that top of the resulting structure (not shown) is completely flat. Thereafter, an array of M.times.N empty slots (not shown) is created on the resulting structure by using a photolithography method, each of the empty slots extending from top of the resulting structure to top of the distal end of each of the actuating structures 100.
After the above step, a mirror layer (not shown) made of a light reflecting material, e.g., Al, and a thin film dielectric layer (not shown) are, sequentially, deposited on top of the sacrificial material including the empty slots, and then the mirror layer and the thin film dielectric layer are, respectively, patterned into an array of M.times.N mirrors 190 and an array of M.times.N thin film dielectric members 195 by using a photolithography or a laser trimming method, thereby forming an array of M.times.N semifinished actuated mirrors (not shown), wherein each of the mirrors 190 has a recessed portion 197 which is attached on top of the distal end of the actuating structure 100.
The preceeding step is then followed by completely covering each of the semifinished actuated mirrors with a second thin film protection layer (not shown).
The sacrificial material is then removed by using an etching method. Thereafter, the second thin film protection layer is removed, thereby forming the array 200 of M.times.N thin film actuated mirrors 201, as shown in FIG. 1I.
There are certain deficiencies associated with the above described method for manufacturing the array 200 of M.times.N thin film actuated mirrors 201. For example, the method involves a number of high temperature processes, especially during the early stages, e.g., the formation of elastic layer 140 made of a nitride requiring a minimum temperature of 800.degree. C., and the active matrix 110 usually is not able to withstand such a high temperature, resulting in a thermal damage thereof.